A charge trapping nonvolatile memory, which memorizes data by trapping a charge in an insulating film, is known as a nonvolatile semiconductor memory device that can electrically program and erase data. For example, a MONOS-type memory is known as such a memory, in which a transistor of MONOS (metal electrode/oxide film/silicon nitride film/oxide film/semiconductor substrate) structure is used for the memory cell transistor. In this MONOS-type memory, the threshold of the cell transistor is controlled by injecting an electron into the silicon nitride film via the oxide film formed on the semiconductor substrate to trap the electron at the charge capture position, or by injecting a hole to annihilate the trapped electron, and thereby data can be stored.
In this charge trapping nonvolatile memory, since the charge storage unit is an insulating film, there is little interference with an adjacent cell due to capacity coupling between charge storage units. Therefore, the charge trapping nonvolatile memory, which uses an insulating film as the charge storage unit, is suitable for shrinking of cell area compared to a floating gate nonvolatile memory, which has a conductive floating gate as the charge storage unit.
However, in this charge trapping nonvolatile memory, a gate insulating film of the same structure as the memory cell transistor is used also for a selection transistor that selects a memory string. Therefore, a charge storage film is provided also in the gate insulating film of the selection transistor. Accordingly, if reading operation is performed a number of times, a charge is injected into the charge storage film of the selection transistor due to the voltage stress applied to the gate electrode of the selection transistor at the time of reading operation, and this results in a fluctuation in the threshold of the selection transistor over time, which may cause the charge trapping nonvolatile memory to malfunction.